Coatings and other material layers can serve a protective function when disposed on a substrate. These layers and coatings are often designed to prevent damage to the subjacent material. This damage can arise when the coating is abraded, impacted, scratched, gouged, and otherwise contacted in a manner deleterious to the material layer. Corrosion is also damaging, whereby materials (particularly metals) are deteriorated at their surfaces due to environmental influences such as, for example, aqueous exposure or immersion. Coatings are also used, most often in the form of paint, to combat and prevent corrosion.
Nonetheless, no coating is perfect and all coatings feature limited hardness, substrate adhesion, and environmental (chemical) durability. Consequently, any coating approach to prevent damage to materials, such as damaged caused by corrosion requires expensive maintenance procedures of periodic stripping and re-painting. Given the prevalence in our society of corrosion and associated protective coatings, the problem and its treatment can be very expensive.
Some solutions that prevent corrosion and related degradation issues utilize compositions that promote self-healing, and in particular self-healing of cracks, gouges, and gaps caused by deformation of the composition. These materials often comprise capsules of various shapes and sizes that are dispersed in a matrix-type material. These capsules contain various materials including monomers or liquid healing agents and catalysts, which are released in response to the destruction of the capsules at the location of the deformation of the coating. The released materials chemically interact with each other, as well as with the catalysts within the matrix material, to cause chemical reactions such as polymerization that can effect “healing” of the coating.
While such solutions can be used and formulated to provide an active, self-healing material, there are limitations to this capsule-based technology. For example, the capsule-based materials do not utilize an active crack closure mechanism. Rather these materials typically rely on the polymerization reactions to fill the deformation in the coating, thus facilitating closure of the crack. It is also likely that a single deformation (e.g., an abrasion or a gouge) in the material can effectively “heal” only once because the capsules are individually formed and located at discrete positions within the matrix. In other words, there are only a finite number of capsules available within the material matrix to promote the healing mechanism, in effect creating a coating that is non-reversible and incapable of repeated healing.
Moreover, the capsules themselves have a finite lifespan that is subject to decay over time. This limitation not only reduces the effective lifespan of the self-healing mechanism, but also reduces the effectiveness of manufacturing and application of batches of the capsule-based materials for use as, e.g., coatings, because the materials that utilize the capsules will only be effective if such manufacturing, application, and re-application is done within the life-span of the capsules, and necessarily the shelf-life of the product.
There is therefore a need for protective coatings that can be more easily repaired to their protective integrity, but that are suited for repeated deformation over a longer life-span. It is also desirable that embodiments of such protective coatings allow for the mending of scratches and cracks by virtue of their composition and microstructure so as to yield a combination of shape memory and rebonding that can be activated by a treatment, e.g., heat treatment.